Refactor ReplicationManager to consider maintenance states

In its current form the replication manager does not consider decommission or maintenance states when checking if replicas are sufficiently replicated. With the introduction of maintenance states, it needs to consider decommission and maintenance states when deciding if blocks are over or under replicated.

It also needs to provide an API to allow the decommission manager to check if blocks are over or under replicated, so the decommission manager can decide if a node has completed decommission and maintenance or not.